Molten metal containment vessels including furnaces, tundishes, ladles and/or crucibles, and other means for holding certain ferrous and non-ferrous metals are well known. The most thermally efficient of these vessels use high quality insulating refractories in the furnace walls to reduce heat losses through those walls. Lids and covers for such vessels have been designed to minimize heat loss through their intrinsic design and the use of effective insulation. Many such vessels, however, require repeated access to the molten metal bath. Access is most frequently needed to withdraw molten metal from the vessel to, for example, feed a castings system.
In such systems, a ladle is often used to withdraw metal from the vessel and transfers this metal to a down sprue, shot sleeve, or some other entry point to the casting process. Such access to molten metal is most often provided through a free surface of molten metal, although other means for metal withdraw, such as a pump, have also been used. Pumps can be expensive, complicated to operate, and may not provide metal withdraw with a geometry that best suits the casting process. The most frequently encountered means for withdrawing molten metal from such vessels is simply dipping a ladle through a free surface of molten metal and filling the ladle.
A free (or exposed) surface of molten metal results in a high rate of heat loss from radiation, conduction and convection heat transfer mechanisms. This is illustrated in the table below, wherein heat loss rates from a 5,000 lb capacity holding furnace for molten aluminum are provided.
Dissipation Estimate, TM = 1350° F.AreaTsnsqPSurfaceft2° F.BTU/Ft2-hr-° F.BTU/hrkWSides40.251303.059,8212.9Ends24.501303.055,9781.8Bottom31.001303.007,4402.2Top56.001703.1020,8326.1Dip out well3.0013503.1028,5048.4Total72,57521.3TAmbient = 50Tm = 1350ε (aluminum) = 0.3
Combined heat loss rates for the walls, bottom, and top of the furnace are 13 kW. By comparison, the dip out well of the furnace is a free surface of aluminum that results in a heat loss of 8.4 kW. Regardless of the effectiveness of the insulation used or design of the furnace sidewalls, ends and bottom, the free surface of molten metal results in an 8.4 kW drop or roughly 40% of the total heat loss.
It is known to provide such metal containment assemblies with some sort of cover to minimize heat losses from the free molten metal surface. For instance, Simko U.S. Pat. No. 5,234,660 shows a ladle cover assembly made from insulating ceramic fiber mats or “batting”. Miller et al. U.S. Pat. No. 4,524,702 shows a supposedly readily repairable, lightweight ceramic fiber cover, over a metal mesh under frame, for heated vessels such as a soaking pit.
It is also known to employ mechanically retractable components in the casting of metal parts. One representative example is the pouring of molten metal through a retractable shield for pour control as taught by Tabatabaci et al. U.S. Pat. No. 6,591,895.